In a gas turbine engine, a fuel and air mixture is ignited for generating combustion gases from which energy is extracted for producing power, such as thrust for powering an aircraft in flight. In one aircraft designated High Speed Civil Transport (HSCT), the engine is being designed for powering the aircraft at high Mach speeds and high altitude conditions. And, reduction of exhaust emissions from the combustion gases is a primary objective for this engine.
More specifically, conventionally known oxides of nitrogen, i.e. NO.sub.x, are environmentally undesirable and the reduction thereof from aircraft gas turbine engines is desired. It is known that NO.sub.x emissions increase when cooling air is injected into the combustion gases during operation. However, it is difficult to reduce the amount of cooling air used in a combustor since the combustor itself is typically made of metals requiring suitable cooling in order to withstand the high temperatures of the combustion gases.
In a typical gas turbine engine, a compressor provides compressed air which is mixed with fuel in the combustor and ignited for generating combustion gases which are discharged into a conventional turbine which extracts energy therefrom for powering, among other things, the compressor. In order to cool the combustor, a portion of the air compressed in the compressor is bled therefrom and suitably channeled to the various parts of the combustor for providing various types of cooling thereof including conventional film cooling and impingement cooling. However, any air bled from the compressor which is not used in the combustion process itself decreases the overall efficiency of the engine, but, nevertheless, is typically required in order to suitably cool the combustor for obtaining a useful life thereof.
One conventionally known, advanced combustor design utilizes non-metallic combustor liners which have a higher heat temperature capability than the conventional metals typically utilized in a combustor. Non-metallic combustor liners may be conventionally made from conventional Ceramic Matrix Composite (CMC) materials such as that designated Nicalon/Silicon Carbide (SIC) available from Dupont SEP; and conventional carbon/carbon (C/C) which are carbon fibers in a carbon matrix being developed for use in high temperature gas turbine environments. However, these non-metallic materials typically have thermal coefficients of expansion which are substantially less than the thermal coefficients of expansion of conventional superalloy metals typically used in a combustor from which such non-metallic liners must be supported.
Accordingly, during the thermal cycle operation inherent in a gas turbine engine, the various components of the combustor expand and contract in response to heating by the combustion gases, which expansion and contraction must be suitably accommodated without interference in order to avoid unacceptable thermally induced radial interference loads between the combustor components which might damage the components or result in an unacceptably short useful life thereof. Since the non-metallic materials are also typically relatively brittle compared to conventional combustor metallic materials, they have little or no ability to deform without breakage. Accordingly, special arrangements must be developed for suitably mounting non-metallic materials in a conventional combustor in order to prevent damage thereto from radial interference during thermal cycles and for obtaining a useful life thereof.
Since non-metallic materials being considered for use in a combustor have higher temperature capability than conventional combustor metals, they may be substantially imperforate without using typical film cooling holes therethrough, which therefore reduces the need for bleeding compressor cooling air, with the eliminated film cooling air then reducing NO.sub.x emissions since such air is no longer injected into the combustion gases downstream from the introduction of the original fuel/air mixture. However, it is nevertheless desirable to cool the back sides of the non-metallic materials in the combustor, with a need, therefore, for discharging the spent cooling air into the flowpath without increasing NO.sub.x emissions from the combustion gases.
Furthermore, the various components of a conventional combustor must also typically withstand differential axial pressures thereon, and vibratory response without adversely affecting the useful life of the components. This provides additional problems in mounting non-metallic materials in the combustor since such mounting must also accommodate pressure loads and vibration of the components in addition to accommodating thermal expansion and contraction thereof.